For much of his early life on Bhola, an island in south-central Bangladesh, Mohammed Abul Kalam battled poverty and a hostile river that twice engulfed his homestead.

Now, as a resident of a "bastee," or private slum on the western edge of the capital, Dhaka, he faces new challenges: the trade-offs he has made on the family's health, education and security in exchange for being near a source of work.

"I came here because I found no other way," Kalam said, sitting on the floor of his tin shack.

The story of how he swapped rural farming for survival in one of Dhaka's burgeoning slums on privately-owned land reflects the precarious situation of up to half a million Bangladeshis estimated to migrate to the capital each year.

Kalam's journey began when the Meghna River wiped out his home for the second time, sending the family deep into debt after he borrowed $765 from moneylenders to build a new house.

His neighbors told him, "Go to Dhaka", suggesting that in order to pay to marry off two teenage daughters, he would have to leave his home in Madras, on Bhola, home to more than two million people, a third of whom live below the poverty line.

With empty pockets, he and his family set off on the 18-hour trip by river to the capital, where he was taken on by a garment washing factory to carry clothes in a role that was a far cry from his old life paddy farming in his village.

Earning just $76 a month, Kalam struggled to make ends meet and, four months into the job, he left to take up other work demolishing buildings with a hammer and a shovel, he said.

This paid a little over $6 a day but the work was irregular and eventually he had no alternative but to find work for his two eldest daughters with a garment producer in Mirpur district.

There, his teenage daughters cut sewing threads and checked clothes for alterations for $51 a month - less than the industry minimum wage of $68.

Rural exodus

Kalam and his family are not alone. According to the World Bank, each year up to half a million rural migrants stream into Dhaka for work, swelling the ranks of the urban poor.

Experts say more than three-quarters of new arrivals end up living in a bastee - owned by private landlords who provide some services - as squatter settlements on public land have disappeared amid demolitions and evictions by authorities.

Since Bangladesh declared independence in 1971, the city's population has quadrupled to around 20 million. By 2050, it is projected to reach more than 35 million.

Three years on, life for Kalam and his family is far from comfortable. He and his wife sleep on the concrete floor of their one-room shack to leave space for four children who share a bed. The family shares a toilet with 10 households and risk fire by cooking with an electric stove as they have no gas.

Even a brief burst of rain sends water into the bastee, which is spread out over five acres of low-lying land.

"[My] sorrows have a beginning but no end. I have lost everything, but the greatest loss is my daughters' education," said Kalam, reflecting on his life in the city.

Private slums

The family rents their room for $32 a month and the landlord takes care of some services, including electricity and water - important in a city where slum-dwellers on public land often have to pay "mastaans", powerful local figures, for utilities.

Experts say the failure to re-house inhabitants evicted from homes in public settlements that have been demolished partly accounts for the boom in privately-owned slums.

"Slums are being cleared, but slum dwellers stay behind - they are not leaving Dhaka," Khondker Rebaca Sun-Yat, executive director at advocacy group the Coalition for the Urban Poor (CUP), told the Thomson Reuters Foundation.

A 2014 census found that nearly 60 percent of slums in the north and south of Dhaka are built on private land, but urban experts and rights groups estimate the figure at 80-90 percent.

Sun-Yat blamed centralized development that focuses services and industries in urban areas for the rise in private slums.

"Cities have sources of income. You build infrastructure in cities; how can you expect rural people not to come to cities?" she said. "If rural areas had income sources and mills and factories, people wouldn't have come to Dhaka," she said.

Nevertheless, she warned that cities would "become paralyzed" if slum-dwellers returned to their place of origin.

The development of Dhaka reflects a wider rise in the numbers of urban poor and what economists call the "non-monetary" conditions of poverty, such as overcrowding, vulnerability, poor security and poor sanitation, experts say.

In comparison to rural poverty, urban poverty is surging.

The number of urban poor in Bangladesh rose to 8 million from 6 million between 1991 and 2010, the latest period for which data is available. In contrast, the number of rural poor went down in the same period, to 46 million from 55 million.

Nine in 10 slum-dwellers in Dhaka were born outside the capital, while one-fifth are poor, according to initial results of a 2016 urban slum survey conducted by the World Bank.

Tenure in privately-owned slums is no more secure than in public squatter settlements, according to Salma A. Shafi, treasurer of the Centre for Urban Studies, a think tank in Dhaka.

"The tenants (in private slums) have no security as rents are raised according to the owner-developers' whims," she said.

"Without any contractual agreement or legal support, tenants have no power."

‘Sub-human conditions’

Mosharraf Hossain, Minister of Housing and Public Works, is among those who believe migration to urban areas of Bangladesh is now "unnecessary" as wages have risen in rural areas.

He said the city was not in a position to absorb more rural migrants given the poor state of its sewerage network, which covers just two-fifths of the city's population.

"It's better not to have slums," Hossain told the Thomson Reuters Foundation at his ministerial office in central Dhaka. "Slum people are living in sub-human conditions, near the rail lines. This is unnecessary."

The government was piloting a low-cost housing project in Mirpur, which would be scaled up if successful, he said.

Kalam said he was prepared to move to another private slum nearby - even for more rent - if he had to, but he did not want to leave Mirpur, where he and his daughters earn their living.

"I never expected my daughters to support me," he said. "Instead, I dreamed they would continue their education."